The Field of Geology— McGee. 193 



the earth's strata brought about by the action of percolating water, 

 air and other gases, the rise of the isogeotherms beneath areas of 

 deposition, the heat of deformation, etc. These processes have af- 

 ffected the rocks ever since the solidification of the planet, but prob- 

 ably with progressively diminishing intensity; by them the rocks ex- 

 posed to degradation are disintegrated, decomposed and softened, and 

 degradation is thereby accelerated; by them the soft sediments are 

 first lithified and then (sometimes) subsequently metamorphosed; 

 and by them the chemic complexity and structural heterogeneity of 

 the terrestrial crust have been largely produced. The various pro- 

 cesses are processes of alteration; and they comprehend lithifaction 

 and its antithesis (decomposition and disintegration combined) 

 in its various phases, or rock formation and rock destruction. 



There is another subordinate category of processes which are 

 intimately allied to the second great category, viz: glacial ion. Only 

 two clearly defined periods^of extensive glaciation (both late Ter- 

 tiary or Quaternary) have been recognized, though others have 

 been suggested; in general the tendency is to obliterate surface 

 irregularity both by grinding down elevations and filling up de- 

 pressions, and thus to perfect the geoid; but glaciation may also 

 accentuate pre-existing irregularities of surface, certainly by mo- 

 raine-building and probably by basin-cutting, and must .therefore 

 be set apart as a unique agency in the modification of the external 

 configuration of the globe. The general process comprises glacial 

 construction and glacial destruction. 



The fourth subordinate category includes the effects of aerial 

 circulation directly upon the land and indirectly throuojh wave and 

 current action. The processes have been in operation throughout 

 geologic time, but so indolently that little traces of their products 

 are found save on the present surface; in general the tendency is 

 to reduce elevations and fill depressions, and thus to merge into 

 gradation; but there is also a tendency to build dunes, beaches and 

 banks, and thus to produce certain minor irregularities of the 

 earth's surface as well as to perpetuate others. The general pro- 

 cess may be called eolation; and its subordinate processes are, like 

 those of the other categories, antagonistic. 



There is a final category which is in part allied to alteration, 

 but is in part unique, viz : the chemic, mechanical and dynamic 

 action of organic life. Ever since the terrestrial crust became so 

 stable as to retain a definite record of the successive stages of world- 



